Phosphor, cathode-ray tube, fluorescent lamp and radiation intensifying screen

ABSTRACT

Disclosed is a phosphor suitable for use in a cathode-ray tube, a fluorescent lamp, a radiation intensifying screen, which comprises transparent spherical particles having an average particle size of 0.5 to 20 μm and a ratio of the major diameter to the minor diameter of individual particles in the range of 1.0 to 1.5, and ultrafine particles having a diameter of 0.2 μm or less in an amount of 5 wt % or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a spherical phosphor, and a cathode-raytube, a fluorescent lamp and a radiation intensifying screen using thesame.

2. Description of the Related Art

Phosphors for use in cathode-ray tubes, fluorescent lamps, or radiationintensifying screens must have a particle size of several μm to obtainsufficient luminous efficiency when a phosphor is excited by electronbeams, ultraviolet radiation or radioactive rays. In order to obtaincrystalline particles of several μm in size, phosphor particles aregenerally synthesized by a solid-phase reaction using a flux. However,the phosphor particles synthesized by the method using the flux are nota completely spherical but near polyhedral shape reflecting crystallinestructures of raw materials and/or the resulting phosphors.

When a phosphor screen of a cathode-ray tube, for example, is formedusing the polyhedral phosphor, it accompanies the drawback that emissiongenerated by electron beam excitation is not fully utilized as lightoutput from the phosphor screen. More specifically, if the shape of thephosphor particles is near polyhedral, a dense phosphor layer free fromvoid cannot be obtained. In addition, an aluminum backing serving as areflecting film formed onto the phosphor layer is inferior insmoothness, exhibiting a rough surface. Consequently, the scattering ofthe emitted light increases causing a loss of light output. In a similarway, if the above phosphor is employed in a fluorescent lamp, emissionby the ultraviolet excitation cannot be efficiently utilized since adense phosphor layer is not obtained.

A color cathode-ray tube is manufactured by, for example, the followingmethod. First, the inner surface of glass is coated with a slurrycontaining a phosphor and a photosensitive resin, thereby forming aphosphor layer. Subsequently, a desired portion alone is hardened byexposure with ultraviolet radiation. Thereafter, a non-exposed portionof the phosphor layer is washed away. If light scattering of thephosphor layer is large, the ultraviolet ray cannot penetrate deep intothe inside portion. As a result, the inside portion is hardly hardenedand thus it is hard to form a phosphor layer thick enough to exhibitmaximum brightness. If light is scattered excessively, it will be alsodifficult to make a phosphor layer pattern into a predetermined shapesince the portion other than the desired portion is hardened byexposure.

In a cathode-ray tube for use in a projection television, a phosphorlayer is generally formed by the following steps. First, phosphorparticles are suspended in an aqueous barium acetate solution placed ina glass bulb for cathode-ray tube. To the suspension solution, anaqueous solution of potassium silicate is added and the phosphorparticles are allowed to settle onto the inner surface of the glassbulb. Three cathode-ray tubes emitting red, green and blue,respectively, are produced in such steps. Images are magnified by meansof three optical lenses individually set in front of the threecathode-ray tubes emitting three colors, and then projected onto ascreen. To provide sufficient brightness levels in the screen, ahigh-power electron gun is used. Even a small defect present on thephosphor layer is magnified clearly on the screen. The dense phosphorscreen, therefore, is strongly demanded with the recent tendency towarda high quality image. Furthermore, even under a high load of electroninput, it is necessary to minimize a decrease of light output anddeterioration.

In most X-ray image intensifiers for use in medical diagnosis and inmaterial examination, an image in the output screen is usually picked upwith a TV camera and amplified for observation. To meet such usages, auniform, dense, and high resolution phosphor screen is required.

The larger the total surface area of phosphor particles contained in thephosphor layer, the more prominent the light scattering. It is thereforedesirable that phosphor particles be as spherical as possible. To formspherical phosphor particles, an emulsion method may be employed whichis disclosed in B. C. Grabmaier, W. Rossner, J. Leppert; Phys. Stat.Sol. (a) 130, K183 (1992). However, the phosphor obtained by this methodis a cluster of fine particles having poor crystalline characteristics,so that the phosphor has to be subjected to refiring. However, theresultant phosphor particles do not always have a completely sphericalshape. In addition, their sizes are small. Hence, they are not suitablefor use in a cathode-ray tube and a fluorescent lamp.

As another method of forming a spherical phosphor particles is disclosedin Jpn. Pat. Appln. KOKAI Publication No. 62-201989. In this method, astarting phosphor consisting of granulated secondary particles is heatedin high-temperature plasma. However, the phosphor formed by this methodhas drawbacks below: It is difficult to obtain a phosphor having apreferable size by this method. In addition, obtained phosphor hasdisadvantageous dispersion and adhesion properties. Further, obtainedphosphor cannot have a desirable activator concentration suitable forpractical phosphor in view of emission color and luminous efficiency.

In a color display, phosphors emitting three colors, red, green, blueare used. A deep red-emitting, bright phosphor is desired for the reasonof broadening a color reproduction range as much as possible. Arepresentative red-emitting phosphor for use in a cathode-ray tube,which almost satisfies the above condition is Y₂ O₂ S:Eu. However, theparticles of Y₂ O₂ S:Eu are polyhedral, so that Y₂ O₂ S:Eu is not freefrom a drawback attributed to the light scattering explained above.Hence, it is strongly demanded that the deep red-emitting phosphorhaving a spherical shape be developed.

A fluorescent lamp for lightning requires not only a sufficient level ofbrightness, namely luminous efficiency, but the level of making anobject color look natural under illumination with a lamp, namely colorrendering properties, so that a three-component type fluorescent lampimproved in both luminous efficiency and color rendering properties maybe widely used. The three-component type fluorescent lamp can beobtained by mixing the following three phosphors in appropriate amountsand coating the phosphor mixture onto the inner surface of a glass tube:

a blue-emitting phosphor having an emission peak near 450 nm such as adivalent europium-activated barium magnesium aluminate phosphor or adivalent europium-activated barium calcium strontium halophosphatephosphor;

a green-emitting phosphor having an emission peak near 545 nm such as acerium terbium-activated lanthanum phosphate phosphor or a ceriumterbium-activated magnesium aluminate phosphor; and

a red-emitting phosphor having an emission peak near 611 nm such as aneuropium-activated yttrium oxide phosphor (Y₂ O₃ :Eu).

An average color rendering index, R_(a) of the fluorescent lampthus-obtained is as high as 84 to 88. The three-component typefluorescent lamp having this value is excellent in appearing the colorof an irradiated object more natural and beautiful. However, thethree-component type fluorescent lamp is disadvantageous in that thespecific color rendering index R₉ for a red color with high chroma is aslow as 20 to 40.

To overcome this problem, the present applicants disclosed a techniquein Jpn. Appln. KOKAI Publication No. 5-244878. This technique is that adeep red-emitting europium-activated monoclinic gadolinium oxide (Gd₂ O₃:Eu) phosphor having an emission peak near 623 nm is blended with theaforementioned three phosphors. By adding the monoclinic Gd₂ O₃ :Euphosphor to the three phosphors in an amount of 12 wt %, R₉ issuccessfully improved by 18 points. On the other hand, total luminousflux, however, decreased by 2.4%. To increase R₉ by 10 points, the totalluminous flux is inevitably decreased by approximately 1.3%.

Gd₂ O₃ :Eu may belong to a monoclinic crystalline system, as shown in R.C. Ropp: J. Electrochem. Soc., Vol. 112, p. 181 (1965). Gd₂ O₃ :Eu isstable in a cubic system at room temperature, as shown in R. S. Roth etal.: J. Res. National Bureau of Standards, Vol. 64A, p. 309 (1960). Inorder to obtain a monoclinic system stable at high temperatures, it isnecessary to heat Gd₂ O₃ to high temperature of 1200° C. or more,followed by quenching. Therefore, it is difficult to prepare themonoclinic system by a usual method of firing in a crucible.

On the other hand, as shown in Arai et al.:J. Alloys and Compounds, Vol.192, p. 45 (1993), since a praseodymium-activated monoclinic Gd₂ O₃ hasa green emission band which cannot be obtained in the cubic Gd₂ O₃, itmay be applied to the usage requiring short persistent green emission.In this case too, it is necessary to overcome the problems in connectionwith preparing a stable monoclinic system in high temperatures.

In recent years, a fluorescent lamp has been frequently used as a backlight for a liquid crystal display. In this case, the fluorescent lampis used in combination with a reflecting film and a light guide plateand a scattering plate. For saving energy, the luminous efficiency isdesired to be as high as possible, when the fluorescent lamp is used incombination with the reflecting film. In a conventional fluorescentlamp, due to low transmittance of a phosphor layer, problematic loss oflight occurs during a process in which part of emission light isreturned into the fluorescent lamp by a reflecting film, transmitsthrough the lamp and converges in one direction. The tube diameter ofthe back light for a liquid crystal display is usually set to asignificantly small value as compared to that (25 to 35 mm) of a lampfor general lighting, taking brightness and compactness intoconsideration. In such a fluorescent lamp, phosphor is coated by meansof the syringe injection method or the sucking method under reducedpressure, not by the slurry flow method employed for a conventionallamp. In this case, if phosphor particles are aggregated in the slurryand a slurry has poor fluidity, an injection nozzle may be clogged withthe aggregated phosphor particles, and a phosphor layer to be formed mayhave a rough screen.

In the case of a radiation intensifying screen, in order to prevent adecrease in the sensitivity, it may be effective to increase radiationabsorption and luminous efficiency by thickening the phosphor layer.However, the thick phosphor layer increases light scattering, with theresult that the sufficient sensitivity cannot be obtained. On the otherhand, when an average particle size of the phosphor particles used inthe phosphor layer is increased, the light scattering can be suppressedbut, instead, sharpness of the obtained radiation image is lowered. Toobtain an intensifying screen having high sensitivity and creating asharp radiation image, a method of forming a double-layered phosphorlayer by coating phosphor particles having different average particlesizes is used (Jpn. Pat. Appln. KOKAI Publication No. 1-57758). In thismethod, first, particles are prepared by a wet precipitation and firingmethod. The obtained particles are classified into two types of phosphorparticles having different average particle sizes (e.g., CaWO₄ of 4.2 μmand 9.6 μm in average particle size) by means of a the sedimentationmethod. To a mixture containing two types of phosphor particlesthus-obtained, a binder is added, thereby making a slurry. Thereafter,the slurry is coated on a protection film by means of a knife coater andsuccessively a slurry containing phosphor particles of a smaller averageparticle size (e.g., CaWO₄ of 4.2 μm in average particle size) alone iscoated on the above phosphor layer in the same manner as above. Onto theresultant phosphor layer, a screen base is adhered, thereby forming theintensifying screen. However, this manufacturing process requires toomany steps and has a drawback in that it is difficult to set sizes andcontents of the phosphor particles since the phosphor particles havingdifferent average particle sizes are used. Due to the aforementioneddrawbacks, it is difficult to obtain a desired radiation intensifyingscreen.

SUMMARY OF THE INVENTION

The present invention made with the view to solve the above-mentionedproblems. An object of the present invention is to provide a phosphor ofa near spherical shape having small particle size. An another object ofthe present invention is to obtain a cathode-ray tube and a fluorescentlamp excellent in brightness, and a radiation intensifying screen havinghigh sensitivity and sharpness by forming a dense and uniform phosphorlayer.

The phosphor of the present invention comprises transparent sphericalparticles having an average particle size of 0.5 to 20 μm and the ratioof the major diameter to the minor diameter of individual particles inthe range of 1.0 to 1.5, and ultrafine particles having a size of 0.2 μmor less in an amount of 5 wt % or less.

In the present invention, phosphors satisfying the above conditions andexhibiting excellent characteristics include:

a rare earth oxide phosphor represented by the following formula:

Ln₂ O₃ :R

wherein Ln is at least one element selected from the group consisting ofLa, Gd, Lu and Y, and R is at least one element selected from thelanthanide group;

a rare earth oxysulfide phosphor represented by the following formula:

Ln₂ O₂ S:R

wherein Ln is at least one element selected from the group consisting ofY, La, Gd, and Lu, and R is at least one element selected from thelanthanide group; and

a tungstate phosphor represented by the following formula:

MWO₄

wherein M is one element selected from the group consisting of Ca andMg; or

CaWO₄ :Pb.

The cathode-ray tube of the present invention comprises a glass tube,the inner surface of which a phosphor screen is formed, and an electronbeam source, wherein at least a part of the phosphor constituting thephosphor screen comprises transparent spherical particles having anaverage particle size of 0.5 to 20 μm and the ratio of the majordiameter to the minor diameter of individual particles in the range of1.0 to 1.5, and ultrafine particles having a diameter of 0.2 μm or lessin an amount of 5 wt % or less. More specifically, the cathode-ray tubemay be used for a direct-view color cathode-ray tube having a shadowmask or a projection television. Further, the cathode-ray tube accordingto the present invention includes a radiation (e.g., X-ray) imageintensifier.

The fluorescent lamp of the present invention comprises a glass tube,the inner surface of which a phosphor layer is formed, and electrodesfor discharging, wherein at least a part of the phosphor constitutingthe phosphor layer comprises transparent spherical particles having anaverage particle size of 0.5 to 20 μm and the ratio of the majordiameter to the minor diameter of individual particles in the range of1.0 to 1.5, and ultrafine particles having a diameter of 0.2 μm or lessin an amount of 5 wt % or less.

The radiation intensifying screen of the present invention comprises aphosphor layer and a protection layer on a screen base, wherein at leasta part of the phosphor constituting the phosphor layer comprisestransparent spherical particles having an average particle size of 0.5to 20 μm and the ratio of the major diameter to the minor diameter ofindividual particles in the range of 1.0 to 1.5, and ultrafine particleshaving a diameter of 0.2 μm or less in an amount of 5 wt % or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a structure of a color cathode-ray tubeaccording to the present invention;

FIG. 2 is a view showing a structure of an X-ray image intensifieraccording to the present invention;

FIG. 3 is a view showing a structure of a fluorescent lamp according tothe present invention;

FIG. 4 is a view showing a structure of a liquid crystal displayapparatus having a light-guide type back light in which the fluorescentlamp according to the present invention is installed;

FIG. 5 is a view showing a structure of a radiation intensifying screenof the present invention;

FIG. 6 is an electron microphotograph showing a particle structure of arare earth oxide phosphor of Example 1 of the present invention;

FIG. 7 is an electron microphotograph showing a particle structure of arare earth oxide phosphor of Example 4 of the present invention;

FIG. 8 is an electron microphotograph showing a particle structure of arare earth oxide phosphor of Example 7 of the present invention;

FIG. 9 is an electron microphotograph showing a particle structure of arare earth oxide phosphor of Example 10 of the present invention;

FIG. 10 is an electron microphotograph showing a particle structure of arare earth oxysulfide phosphor of Example 11 of the present invention;and

FIG. 11 is an electron microphotograph showing a particle structure of acalcium tungstate phosphor of Example 20 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, the present invention is described in detail.

The phosphor of the present invention comprises transparent sphericalparticles having an average particle size of 0.5 to 20 μm and having theratio of the major diameter to the minor diameter of individualparticles in the range of 1.0 to 1.5, and ultrafine particles of 0.2 μmor less in an amount of 5 wt %.

The average particle size of the transparent spherical particlescontained as a main component is defined as 0.5 to 20 μm. If the averageparticle size is smaller than 0.5 μm or larger than 20 μm, thebrightness of the phosphor screen formed will be lowered. Thetransparent spherical particles contained as the main component have theratio of the major diameter to the minor diameter (an aspect ratio),i.e., the ratio of the longest diameter to the shortest diameter ofindividual particles preferably in the range of 1.0 to 1.5 and have anear spherical shape with no protruded edges. The ratio of the majordiameter to the minor diameter of individual particles is morepreferably in the range of 1.0 to 1.2. The ultrafine particles having adiameter of 0.2 μm or less contained in a ratio of 5 wt % or less basedon that of the transparent spherical particles are mainly attached ontothe surface of transparent spherical particles.

Unlike in the phosphor screen made of conventional phosphor, in thephosphor screen formed of the phosphor mainly comprising the sphericalparticles, since the total surface area of the spherical particles issmall, the light scattering within the phosphor layer is decreased andthe transmittance is improved even if the coating weight is equal toeach other. If the transmittance of the phosphor screen is high, lightproceeding in the opposite direction of an electron beam source (a sideserving to human observation) of the entire emitted light on thephosphor screen will increase, either in a color cathode-ray tube or ina monochrome cathode-ray tube. In addition, since the phosphor particlesin the phosphor screen are almost close-packed, a reflecting metal filmformed on the phosphor screen becomes more smooth. Consequently, lightproceeding in a direction of the electron beam source of the entireemitted light on the phosphor screen can be efficiently reflected. Inother words, the light to be used as light output increases resulting inthe improvement of the brightness. Further, in the case where thephosphor screen for use in a color cathode-ray tube is formed by aphotoprinting method, since the phosphor screen is irradiated to thedeep inside thereof by virtue of high level of light transmittance, aphosphor screen can be formed thicker than that made of conventionalphosphor and the thickness is easily controlled. In addition, since thephosphor layer has high level of light transmittance and density, anexcellent phosphor pattern can be obtained free from irregularity at thepattern edges attributable to light scattering.

As described above, the content of the ultrafine particles present inthe phosphor of the present invention is defined as 5 wt % or less. Thereason of this is as follows: If the content of the ultrafine particlesattached onto the surface of or mixed with the transparent sphericalparticles of several μm exceeds 5 wt %, the transmittance of thespherical particles and the formed phosphor layer decreases since thelight scattering increases. Further, since the ultrafine particles havelow luminous efficiency, if they are mixed with the transparentspherical particles of several μm, the luminous efficiency of the entirephosphor will decrease. In contrast, if the ultrafine particles arepresent in an amount of 5 wt % or less, the light transmittance and theluminous efficiency of the phosphor layer will not decrease. Theultrafine particles attached onto the spherical particles improve thefluidity and dispersion properties of the phosphor and adhesion abilityof the phosphor layer to a substrate. If phosphor having good fluidityis blended with other phosphor, a uniform phosphor layer can be readilyobtained. Hence, the phosphor containing ultrafine particles in anamount of 5% or less are useful particularly in a fluorescent lamp inwhich a phosphor mixture are usually used. If the dispersion isimproved, the packing of the phosphor particles in the phosphor layer,which is formed by the settling method or the slurry method, is furtherimproved. The improvement in adhesion ability is presumably brought bythe function that the ultrafine particles act as a low-melting pointbinder in a baking step performed at 400° to 700° C., in manufacturingprocess for a fluorescent lamp and a cathode-ray tube. However, if theultrafine particles are present in an amount of 0.001 wt % or less, theabove advantages may disappear.

A suitable average particle size of the spherical particles and asuitable content of the ultrafine particles vary depending on the typesof phosphors and the application thereof. Hence, hereinbelow, we willindividually explain them in detail with reference to a rare earth oxidephosphor, a rare earth oxysulfide phosphor and a tungstate phosphor.

The rare earth oxide phosphor of the present invention is represented bythe following formula:

Ln₂ O₃ :R

wherein Ln is at least one element selected from the group consisting ofLa, Gd, Lu and Y, and R is at least one element selected from thelanthanide group. The rare earth oxide phosphor of the present inventioncomprises transparent spherical particles having an average particlesize of 0.5 to 15 μm and having the ratio of the major diameter to theminor diameter of individual particles in the range of 1.0 to 1.5, andultrafine particles having a diameter of 0.2 μm or less in an amount of2 wt % or less. R denotes an element of the lanthanide group. Of thelanthanide group, particularly useful elements for a phosphor include:Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb. Further it is desirablethat R be Eu and the molar ratio of Eu to Ln be in the range of 1 to 6%,and R be Tb and the molar ratio of Tb to Ln be in the range of 0.1 to6%; and R be Pr and the molar ratio of Pr to Ln be in the range of 0.01to 0.5%. If the activator content does not satisfy the above range, sucha phosphor cannot be put into practical use in a cathode-ray tube or afluorescent lamp, in view of the emission color and the luminousefficiency. The molar ratio range of Tb is broader than anotheractivators. This is because the phosphor containing Tb in a low amountemits blue and the phosphor containing Tb in a high amount emits green.Both cases have practical usages to meet with their specific features.

A preferable rare earth oxide phosphor is represented by the followingformula:

Gd₂ O₃ :R

wherein R is at least one element selected from the lanthanide group, atleast a part of the crystalline system thereof is a monoclinic system,and comprising transparent spherical particles having an averageparticle size of 0.5 to 15 μm and the ratio of the major diameter to theminor diameter of individual particles in the range of 1.0 to 1.5, andultrafine particles having a diameter of 0.2 μm or less in an amount of0.5 wt % or less.

A Gd₂ O₃ :R phosphor can be preferably used in a cathode-ray tube,however, the application of Gd₂ O₃ :R varies depending on the emissioncolor, red or green which is determined by types of R. For example, inthe case of a monoclinic system Gd₂ O₃ :Eu, the emission color isdeep-red deeper than that emitted by a phosphor of a cubic system stablein low temperature. This red emission is suitably used as a redcomponent in a color cathode-ray tube and a projection cathode-ray tube.In the case of monoclinic Gd₂ O₃ :Pr, the emission color changes toyellowish-green containing a green emission band from red emission of acubic system stable at low temperatures. Since the monoclinic Gd₂ O₃ :Prshows a very short afterglow as about 10 μs, it is suitably used as agreen component in a specific cathode-ray tube required to be shortpersistent. Monoclinic Gd₂ O₃ :Tb shows green emission having high levelof luminous efficiency, so that it is suitable as a green component foruse in a projection CRT. A Gd₂ O₃ :R phosphor for use in such acathode-ray tube is preferred to have an average particle size of 2 to10 μm.

Of the aforementioned Gd₂ O₃ :R phosphors, a phosphor represented by theformula Gd₂ O₃ :Eu containing monoclinic crystallites in an amount of 5to 100% and having a particle size of 0.5 to 3 μm is suitably used as ared component for use in a fluorescent lamp employing a phosphormixture. In this case, it is preferable that the concentration of Eu bein the range of 1 to 6 mol %. If the concentration of Eu deviates fromthe above range, the luminous efficiency will be lowered. The averageparticle size is defined as 0.5 to 3 μm for the following reasons: It isempirically known that when a phosphor mixture is used in a fluorescentlamp, a red-emitting phosphor functions adversely to a total luminousflux from a view point of luminosity, and a total luminous fluxincreases as the phosphor particle size decreases. For this reason, anaverage particle size of the phosphor is preferred to be 3 μm or less.However, when the particle size of the phosphor is excessively small,the phosphor powder is hard to handle. The average particle size is,therefore, preferred to be 0.5 μm or more.

The rare earth oxysulfide phosphor of the present invention isrepresented by the following formula:

Ln₂ O₂ S:R

wherein Ln is at least one element selected from the group consisting ofY, La, Gd, and Lu, and R is at least one element selected from thelanthanide group, and comprising transparent spherical particles havingan average particle size of 0.5 to 15 μm and the ratio of the majordiameter to the minor diameter of individual particles in the range of1.0 to 1.5, and ultrafine particles having a size of 0.2 μm or less inan amount of 2 wt % or less. This phosphor can be used in a cathode-raytube, an X-ray intensifying screen or a ceramics for scintillator. It ispreferable that R be Eu and the molar ratio of Eu to Ln be 2 to 7%, andthat R be Tb and the molar ratio of Tb to Ln be 0.1 to 6%, and that R bePr and the molar ratio of Tb to Ln be 0.01 to 0.5%. If the activatorcontent does not fall within the above range, such a phosphor cannot beput into practical use in the aforementioned application, in view ofemission color and luminous efficiency. The molar ratio range of Tb isbroader than another activators. This is because the phosphor containinga low amount of Tb emits blue light and the phosphor containing a highamount of Tb emits green light. Both cases are practical depending onapplications. This phosphor is preferred, when a powder layer having anoptically indefinite thickness is formed of the phosphor, to havereflectance for visible light of 85% or more. If the reflectance is lessthan 85%, the luminous efficiency will be lowered 10% or more. Such aphosphor is disadvantageous for practical use.

The tungstate phosphor of the present invention is represented by thefollowing formula:

MWO₄

wherein M is one of the group consisting of Ca and Mg, or

CaWO₄ :Pb,

and comprising transparent spherical particles having an averageparticle size of 0.5 to 20 μm and the ratio of the major diameter to theminor diameter of individual particles in the range of 1.0 to 1.5, andthe ultrafine particles having a diameter of 0.2 μm or less in an amountof 0.001 to 5 wt %. In particular, it is preferable that the ultrafineparticles of 0.2 μm or less be contained in the range of 0.01 to 2 wt %.This phosphor can be used in a cathode-ray tube and a fluorescent lamp.The tungstate phosphor having the above particle structure can beimprove in a level of brightness in view of luminosity since theemission spectrum shifts to the longer wavelength side. Further, thedeviation of the excitation peak wavelength from 254 nm becomes small,the excitation efficiency can be improved over a conventional level whenthe phosphor is excited by ultraviolet radiation of 254 nm. As a result,brightness of the phosphor screen is improved.

The phosphor of the present invention can be prepared by fusing startingphosphor particles and then quenching them. To be more specific, amethod can be employed in which the starting phosphor particles aresupplied to a high-temperature thermal plasma with a carrier gas andexpelled out of the thermal plasma in short time. The thermal plasmaused herein indicates a state in which gas is ionized in hightemperatures. The thermal plasma can be generated by gas discharge usinga high frequency electromagnetic wave of several to several tens ofmegahertz or direct current. By this, the gas temperature of theso-called torch or the frame portion reached several thousands to tenthousands degrees. The high frequency thermal plasma apparatus isdetailed, for example, in Yoshida et al., Iron and Steel, vol. 68, No.10, p. 20 (1982).

Unlike in a preparing method disclosed in Jpn. Pat. Appln. KOKAIPublication No. 62-201989, in the present invention a starting phosphoris employed which has an activator concentration different from that ofa desired phosphor, and it is not granulated, consisting of primaryparticles. Such a starting phosphor may be manufactured by firing with aflux or by thermally decomposing a co-precipitate of oxalates. Further,the difference in the average particle size between the startingphosphor and a final spherical phosphor can be lowered less than 50% byimproving dispersion properties and fluidity thereof by treating thesurface of the starting phosphor particles with acid or providing asmall amount of organic surfactant to the surface. A primary particlesize of the starting phosphor is preferably about 2 μm or more. This isbecause that if the primary particle size is smaller, final particlesobtained by vaporization in the thermal plasma followed by quenchingwill have a size of 0.2 μm or less in most case. In contrast, if theprimary particle size or the secondary particle size is excessivelylarge, the final particle size will be too large to put into practicaluse. Hence, the size of the primary and secondary particles is desiredto be 20 μm or less. Even if the primary particle size is 2 μm or more,since a part of the phosphor is vaporized in the plasma treatment andquenched, the final phosphor will contain ultrafine particles having adiameter of 0.2 μm or less. The amount of the ultrafine particles variesdepending on power of the thermal plasma, a supply position of thestarting phosphor, and a recovery method of the treated phosphor. In thepresent invention, when ultrafine particles are present excessively,they are removed by means of ultrasonic treatment performed in a liquid,e.g., water, thereby adjusting the amount of ultrafine particles to 5 wt% or less. After the thermal plasma treatment is completed, when theobtained phosphor is further fired at 800° to 1200° C., aggregatedultrafine particles grow again with increase in particle size and attachonto the surface of the spherical particles.

In the present invention, the crystal system may be made both ahigh-temperature phase and a low-temperature phase. For example, in thecase of the Gd₂ O₃ phosphor, even if the crystalline system of thestarting material is a cubic system stable in low temperatures, thecubic system can be readily converted into the monoclinic system stablein high temperatures through a process in which starting particles areexposed, in thermal plasma, to temperatures higher than the transitiontemperature from the cubic system to the monoclinic system and thenquenched. On the other hand, when the spherical phosphor containingultrafine particles of the present invention is further refired at 800°to 1200° C., the spherical phosphor can be easily converted into a cubicsystem stable in low temperatures, while maintaining the sphericalshape.

In the present invention, an activator concentration of the phosphorobtained by the thermal plasma treatment differs from that of thestarting phosphor. For example, when a red-emitting Y₂ O₃ :Eu phosphorfor use in a lamp is employed as a raw material, even if the molar ratioof Eu/Y of the starting phosphor is 4.4%, the ratio of Eu/Y of thethermal-plasma treated spherical phosphor decreases to approximately3.5%. Whereas, the ratio of Eu/Y of the ultrafine particles reaches toabout 20%. As a result, light emitted from the spherical phosphorexhibits an orange color shifting from a desired red color and theluminous efficiency is reduced by approximately 20%. In the case of aTb-activated monoclinic Gd₂ O₃ phosphor, if a Tb concentrationdecreases, a blue component represented by an emission line at 415 nmincreases with respect to a green component represented by an emissionline at 544 nm in the emission spectrum. In order to obtain a desiredgreen-emitting phosphor, it is necessary to set the molar ratio of Tb/Lnto a value in the range of 2 to 6%. However, the molar ratio of Tb/Ln ofthe obtained phosphor changes from that of the starting phosphor by thethermal plasma treatment. The degree of the change in the activatorconcentration varies depending on conditions of the thermal plasmatreatment, such as the supply amount of the starting phosphor. However,it is impossible to completely eliminate the change in the activatorconcentration. Hence, in order to obtain the activator concentration ofthe spherical phosphor providing a desired emission color, it isnecessary to control the activator concentration of the startingphosphor.

Hereinbelow, the cathode-ray tube, the fluorescent lamp, the radiationintensifying screen of the present invention using various phosphorswill be individually explained.

First, we will describe the cathode-ray tube. Examples of cathode-raytubes include a direct-view color cathode-ray tube, a direct-viewcathode-ray tube for terminal display, a cathode-ray tube for use in aprojection television, a cathode-ray tube for use in an X-ray imageintensifier, a low voltage electron beam fluorescent display and thelike.

For example, a structure of the direct-view color cathode-ray tube isshown in FIG. 1. A glass bulb 1 comprises a panel, a funnel, and a neck.The inner surface of the panel constituting the glass bulb 1, a phosphorscreen 2 is formed. On the phosphor screen 2, a reflecting film (notshown) made of aluminum and the like is formed. On the rear side of thereflecting film, a shadow mask 3 is placed. To the neck of the glassbulb 1, an electron gun 4 is attached. To the outer portion of the neck,a deflecting yoke 5 is provided. The electron beam radiated from theelectron gun 4 is deflected by the deflecting yolk 5 and transmitsthrough the shadow mask 3, and irradiates on the phosphor screen 2. InFIG. 1, a cathode-ray tube has a shadow mask. Whereas, the shadow maskis not provided to a monochrome cathode-ray tube for use in theprojection television or the like.

The phosphor constituting a phosphor screen of the direct colorcathode-ray tube is not particularly restricted as long as the phosphorpossesses the particle structure defined by the present invention andincludes not only a rare earth oxide and a rare earth oxysulfide butzinc sulfide phosphors such as ZnS:Ag, ZnS:Cu, or ZnS:Cu, Au. In thephosphor screen of the direct-view cathode-ray tube formed by thephosphor of the present invention, it is easy to control the thicknessof the phosphor layer and to attain a high level of brightness since thephosphor layer shows low light-scattering and high transmittance. Inaddition, a phosphor layer pattern can be formed in an almost the samepredetermined design. Here, it is preferred that the average particlesize of the transparent spherical particles be 2 to 10 μm and thecontent of ultrafine particles having a diameter of 0.2 μm or less be0.01 to 0.5 wt %.

The phosphor constituting a phosphor screen of the cathode-ray tube forprojection television is not particularly restricted as long as thephosphor possesses the particle structure defined by the presentinvention and includes not only a rare earth oxide and a rare earthoxysulfide but phosphors such as ZnS:Ag, Y₃ (Al,Ga)₅ O₁₂ :Tb, Y₂ SiO₅:Tb, InBO₃ :Tb, Zn₂ SiO₄ :Mn, LaOCl:(Tb, Ti), and (La, Gd)OBr:Ce. Thesephosphors may be mixed with each other, or mixed with conventionalnon-spherical phosphors. The phosphor screen of the cathode-ray tube forprojection television formed by the phosphor of the present inventionexhibits high packing density of the phosphor since the sphericalparticles show good dispersion properties without aggregation. Further,since the phosphor contains the ultrafine particles having a diameter of0.2 μm or less, the phosphor layer has strong adhesion ability to glass.Note that, it is preferred that the average particle size of thetransparent spherical particles be 2 to 10 μm and the content ofultrafine particles having a diameter of 0.2 μm or less be 0.01 to 0.5wt %. Hence, when an image is magnified and projected on a screen, itdoes not make conspicuous the roughness of the phosphor screen. Further,since the heat conductivity of the phosphor layer increases, elevationof temperature and deterioration of the phosphor are low even if anelectron beam input is in a high level, that is, under a high load. Inaddition, an electron beam does not directly irradiate a glasssubstrate, so that the glass is colored little. Hence, light outputdecreases little even if the cathode-ray tube is driven for a long time.

An X-ray image intensifier, which is one of cathode-ray tubes, is shownin FIG. 2. 16 denotes a glass bulb. An input phosphor screen 11 convertsan X-ray input into visible light. A photoelectric cathode 12 releaseselectrons from a position receiving light. Focusing electrodes 13 forman electron lens in the glass bulb 16. An anode 14 creates a potentialdifference of 25 to 30 kV between the photoelectric cathode 12 andaccelerates the electrons released from the photoelectric cathode 12. 15denotes an output phosphor screen, where a phosphor layer and analuminum film are formed successively on glass.

The present invention is directed to a phosphor used in the outputphosphor screen 15. As such a phosphor, a conventional (Zn, Cu)S:Agbased zinc sulfide phosphor may be used. However, this phosphor has adrawback in that when the particle size is set to a value less than 1μm, the luminous efficiency severely decreases. In contrast, the rareearth oxide phosphor and the rare earth oxysulfide phosphor of thepresent invention show a smaller decrease in the luminous efficiency,and therefore can be suitably used. More specifically, by virtue of gooddispersion properties of the spherical phosphor particles, the phosphorof the present invention can provide a uniform phosphor screen having ahigh packing density. Further, owning to a spherical shape, the phosphorparticles contact to glass by almost a point. Due to the point contact,the degree of optical coupling decreases, with the result that theamount of a component emitted in a direction parallel to the glassdecreases as compared to a phosphor having a non-spherical shape.Consequently, resolution, contrast and output are improved. To improvethe resolution, it is desirable that a phosphor has a small particlesize. However, when the phosphor has an excessively small particle size,the luminous efficiency decreases. Hence, it is desirable that theaverage particle size of the phosphor be 0.5 to 3 μm. Further, by virtueof the ultrafine particles of 0.2 μm or less attached onto the sphericalparticles, the phosphor shows sufficient dispersion and fluidity in theformation of the phosphor layer by the settling method, the centrifugalmethod or the electrode position method, and the formed phosphor layerhas strong adhesion ability. The cathode-ray tubes, wherein at least apart of the phosphor constituting the phosphor screen possesses theparticle structure defined by the present invention, show a part of themerits described above, even if the conventional non-spherical phosphorsare used as the counterpart of a phosphor mixture or as color componentsdifferent from that of the phosphor possessing the particle structuredefined by the present invention.

Hereinbelow, the fluorescent lamp according to the present inventionwill be explained. A structure of the fluorescent lamp is shown in FIG.3. In the inner surface of a glass tube 21, a phosphor layer 22 isformed. At the end portions of the glass tube 21, electrodes 23 fordischarge are formed. In the glass tube 21, a rare gas and mercury arecharged. The fluorescent lamp is not restricted to a straight tube, anda circular or a compact-type tube may be used.

The phosphor of the present invention exhibits no aggregation and gooddispersion properties. Since the phosphor of the present inventioncontains ultrafine particles and shows good fluidity, a uniform mixingcan be attained when the phosphor of the present invention is blendedwith two or more types of another phosphors, leading to a dense phosphorscreen. Hence, mismatching in the fluorescent color between both ends ofthe fluorescent lamp is negligible. Here, it is preferred that theaverage particle size of the transparent spherical particles be 0.5 to10 μm and the content of ultrafine particles having a diameter of 0.2 μmor less be 0.01 to 2 wt %. The phosphor constituting a phosphor layer ofthe fluorescent lamp is not particularly restricted as long as thephosphor has a particle structure defined by the present invention. Forexample, blue-emitting phosphors such as CaWO₄, CaWO₄ :Pb, BaMg₂ Al₁₆O₂₇ :Eu, and (Sr, Ca)₁₀ (PO₄)₆ Cl₂ :Eu, green-emitting phosphors such asLaPO₄ :(Ce, Tb), CeMgAl₁₁ O₁₉ :Tb, and Zn₂ SiO₄ :Mn, and red-emittingphosphors such as Y₂ O₃ :Eu, and Y(P,V)O₄ :Eu may be used.

The aforementioned monoclinic Gd₂ O₃ :Eu phosphor has an emission peaknear 623 nm and can be suitably used in a fluorescent lamp utilizing aphosphor mixture. To be more specific, if the phosphor layer is formedby a phosphor mixture mainly consisting of the phosphor of the presentinvention, a red-emitting europium-activated yttrium oxide phosphorhaving an emission peak near 611 nm, a green-emitting phosphor having anemission peak in the range of 540 to 570 nm, and a blue-emittingphosphor having an emission peak near 450 nm, R₉ of the fluorescent lampcan be improved. In this case, the green-emitting phosphor is at leastone selected from the group consisting of a cerium terbium-activatedlanthanum phosphate phosphor and a cerium terbium-activated bariummagnesium aluminate phosphor. The blue-emitting phosphor is at least oneselected from the group consisting of a divalent europium-activatedbarium magnesium halophosphate phosphor, a divalenteuropium-manganese-coactivated barium magnesium halophosphate phosphorand a divalent europium-activated barium calcium strontium halophosphatephosphor. The contents of the above phosphors vary depending on whatdegree the correlated color temperature of the fluorescent lamp is setto. A desired fluorescent lamp can be usually obtained when theblue-emitting phosphor is contained in an amount of 10 to 50 wt %; thegreen-emitting phosphor in an amount of 20 to 45 wt %; a sum of the Gd₂O₃ :Eu phosphor and the Y₂ O₃ :Eu phosphor in an amount within the rangeof 30 to 76 wt %.

Note that, since a cubic Gd₂ O₃ :Eu phosphor has an emission peak near611 nm similar to that of a Y₂ O₃ :Eu phosphor, it can be used in placeof the Y₂ O₃ :Eu phosphor without any adverse effect on the colorrendering properties. Therefore, the Gd₂ O₃ :Eu phosphor of the presentinvention does not necessarily belong to entirely monoclinic system andmay contain cubic system particles as long as it contains the monoclinicsystem in an amount of approximately 5% or more.

The phosphor of the present invention can be suitably used in afluorescent lamp having a tube diameter of 8 mm or less. Since thephosphor particles are excellent in dispersion properties and fluidity,they do not cause clogging of a nozzle when the small-diameterfluorescent lamp is formed by coating the phosphor by means of thesyringe injection method or the sucking method under reduced pressure.Owning to this advantage, a uniform phosphor layer can be easilyobtained.

The fluorescent lamp of the present invention is useful when it is usedin combination with a reflecting film as is in the case of a light-guidetype back light of a liquid crystal display. A structure of the liquidcrystal display having a light-guide type back light in which afluorescent lamp is installed is shown in FIG. 4. 31 denotes afluorescent lamp. 32 is a reflecting film. 33 is a light-guide plate. 34is a diffusion plate. 35 is a liquid crystal display panel. 36 is a lampcover. Light radiated from the fluorescent lamp 31 in directions otherthan the direction toward the light guide plate 33 converges in adirection of the light guide plate 33 being reflected by means of thereflecting film 32. In this case, it is preferable that 1/3 or more ofthe outer surface of a glass tube of the fluorescent lamp be coveredwith a reflecting material having reflectance of 50 to 98%. When thereflecting material covers 1/3 or less or the reflecting material hasreflectance of 50% or less, the light convergence effect is low and itis no use employing the reflecting material. Actually, there is noreflecting material having reflectance in excess of 98%. The most lightreflected by the film 32 satisfying the above conditions transmitsacross the fluorescent lamp 31 and converges in a direction of the lightguide plate 33. The phosphor layer of the fluorescent lamp of thepresent invention has a higher level of transmittance compared to aphosphor layer formed of conventional phosphors, since it is mainlyformed of spherical particles. In the fluorescent lamp of the presentinvention, even if overall light output is equal, light outputconverging in a direction of the light guide plate is larger by 10%compared to the fluorescent lamp using a conventional phosphor. Inaddition, as described above, the fluorescent lamp of the presentinvention can be easily manufactured even if a tube diameter is 8 mm orless. This feature is advantageous in forming a thin liquid crystaldisplay. The fluorescent lamps, wherein at least a part of the phosphorconstituting the phosphor layer possesses the particle structure definedby the present invention, show a part of the merits described above,even if the conventional non-spherical phosphors are used as thecounterpart of a phosphor mixture.

Hereinbelow, the radiation intensifying screen will be explained. Astructure of the radiation intensifying screen is shown in FIG. 5. 41aand 41b are radiation intensifying screens (41a is a front intensifyingscreen and 41b is a back intensifying screen). 42 denotes a photographicfilm. 43 is a radioactive ray which has been transmitted through anobject to be photographed. The radiation intensifying screens 41a and41b comprise a phosphor layer 45 and a protection film 46 mounted on ascreen base 44. The phosphor of the intensifying screens 41a and 41bemits luminescence by input radiation. The luminescence efficientlyirradiates a photographic film on both sides. The phosphor constitutingthe phosphor layer is not particularly restricted as long as thephosphor possesses the structure defined by the present invention.Examples of the phosphor include not only a tungstate phosphor, a rareearth oxysulfide phosphor, and a rare earth oxide phosphor but Ba_(1-x)Sr_(x) FCl_(1-y) Br_(y) :Eu (x=0-1, y=0-1), BaSO₄ :Eu, LaOBr:R (R=Tb,Tm), HfP₂ O₇, Hf₃ (PO₄)₄, YTaO₄, GaTaO₄ and the like.

When a photograph is taken by use of a radiation intensifying screenemploying a phosphor having an average particle size of 20 μm or more, auniform and smooth image is not obtained. It is preferable that theultrafine particles be contained in the phosphor in an amount of 0.01 to2 wt %. As described above, if the phosphor layer is formed by using thephosphor of the present invention, the brightness of the phosphor layerwill be improved. Since sufficient brightness can be attained, thephosphor layer can be thinned, with the result that the radiation imagehaving high level of sharpness can be obtained.

Further, since the phosphor of the present invention contains ultrafineparticles inducing good dispersion properties, a uniform phosphor layercan be formed. Hence, using two types of phosphors whose averageparticle sizes are different, a radiation intensifying screen having adouble-layered phosphor layer can be easily manufactured. In addition,by use of the ultrafine particles contained in the phosphor of thepresent invention, a radiation intensifying screen having thedouble-layered phosphor layer can be manufactured even if phosphorparticles having different sizes are not used. More specifically, themethod comprises the steps of providing a phosphor containing theultrafine particles in a relatively large amount (approximately 1 wt %);dispersing the particles of several μm in size and ultrafine particlesby means of an ultrasonic vibration; and, thereafter, forming a phosphorlayer on a protection film by the settling method. By this method, thelayer made of the ultrafine particles is formed on the side of thescreen base, and a dense layer made of a phosphor mixture consisting ofthe particles of several μm and the ultrafine particles on the formerlayer. In this case, the radiation intensifying screen can be obtainedin less steps than a method employing two types of phosphors havingdifferent sizes. Besides this, the sensitivity and sharpness of theradiation image are improved.

The radiation intensifying screens, wherein at least a part of thephosphor constituting the phosphor screen possesses the particlestructure defined by the present invention, show a part of the meritsdescribed above, even if the conventional non-spherical phosphors areused as the counterpart of a phosphor mixture or as a layer differentfrom that of the phosphor possessing the particle structure defined bythe present invention.

EXAMPLES

Hereinbelow, the Examples of the present invention will be described indetail.

(Example 1)

As a raw material, a conventional non-spherical Y₂ O₃ :Eu phosphor wasused. The average particle size of the starting phosphor was 4.5 μm asmeasured by the Blaine method. The starting phosphor was supplied intohigh frequency plasma with an argon gas serving as a carrier gas, fused,and quenched, thereby obtaining the phosphor according to the presentinvention. The average particle size of the obtained phosphor was 4.8 μmas measured by the Blaine method. An electron microphotograph of theobtained phosphor is shown in FIG. 6. The ratio of the major diameter tothe minor diameter with respect to individual phosphor particles fellwithin the range of 1.00 to 1.10 as measured from the electronmicrophotograph. The X-ray diffraction pattern of the phosphor wasequivalent to that of Y₂ O₃. The powder brightness of the phosphorexcited by an electron beam was 98% based on that of the startingphosphor as measured under an accelerating voltage of 10 kV and acurrent density of 1 μA/cm².

Subsequently, using the obtained phosphor, a phosphor screen was formedwith a coating weight of 7 mg/cm² on the inner surface of a glass bulbby means of the settling method. An aluminum backing was provided and anelectron gun was installed, followed by evacuation and sealing, therebyobtaining a 7-inch projection cathode-ray tube. The brightness of theprojection cathode-ray tube was 790 ft-L as measured under a voltage of30 kV and a beam current of 200 μA. This value was 5% higher than thebrightness, 750 ft-L, of a cathode-ray tube formed in the same manner asabove using the starting phosphor.

(Example 2)

A precipitate obtained by co-precipitating a mixed oxalate was fired todecompose at 900° C., and then the resultant product was fired at 1100°C. using an alkali earth halide as a flux, thereby obtaining a La₂ O₃:Pr phosphor having a Pr concentration of 0.1 mol %. The averageparticle size of the starting phosphor was 6.8 μm as measured by theBlaine method. The starting phosphor was supplied into high frequencyplasma with an argon gas serving as a carrier gas, fused, and quenched,thereby obtaining the phosphor according to the present invention, inthe same manner as in Example 1. The average particle size of theobtained phosphor was 7.3 μm as measured in the Blaine method. The ratioof the major diameter to the minor diameter with respect to individualphosphor particles fell within the range of 1.00 to 1.15 as measuredfrom an electron microphotograph. In addition, the phosphor containedultrafine particles in an amount of 0.3 wt %. The powder brightness ofthe phosphor was 78% based on that of the starting phosphor as measuredin the same conditions as in Example 1. It is presumed that such a lowpowder brightness is ascribed to that Pr was oxidized to some extent.The emission color was green, showing a spectrum having peaks near 510nm and near 670 nm. The emitted color was consistent with that of thestarting phosphor.

Subsequently, using the obtained phosphor, a phosphor screen was formedwith a coating weight of 11 mg/cm² on the inner surface of a glass bulbby means of the settling method. An aluminum backing was provided and anelectron gun was installed, followed by evacuation and sealing, therebyobtaining a 7-inch projection cathode-ray tube. The brightness of theprojection cathode-ray tube was 300 ft-L as measured under a voltage of30 kV and a beam current of 200 μA. This value was 20% higher than thebrightness, 250 ft-L, of a cathode-ray tube formed in the same manner asabove using the starting phosphor. As described above, despite that thephosphor in this Example has low powder brightness as compared to thatof the starting phosphor, the brightness of the cathode-ray tube usingthe phosphor is high. This is because the shape of the phosphorparticles is nearly complete sphere.

(Example 3)

A precipitate obtained by coprecipitating a mixed oxalate was fired todecompose at 900° C., the resultant product was fired at 1400° C.without using a flux, thereby obtaining a Gd₂ O₃ :Eu phosphor having anEu concentration of 5 mol %. The X-ray diffraction of the obtainedphosphor was measured. Although most part of the phosphor was monoclinicGd₂ O₃, a cubic Gd₂ O₃ pattern was also observed in an amount of 5%judging from a comparison of respective maximum peaks. The averageparticle size of the starting phosphor was 3.5 μm as measured by theBlaine method. The phosphor was relatively aggregated. The startingphosphor was supplied into high frequency plasma with an argon gasserving as a carrier gas, fused, and quenched, thereby obtaining thephosphor according to the present invention. The average particle sizeof the obtained phosphor was 4.2 μm as measured by the Blaine method.The ratio of the major diameter to the minor diameter with respect toindividual phosphor particles fell within the range of 1.00 to 1.18 asmeasured from an electron microphotograph. When the X-ray diffractionpattern of the obtained phosphor was measured, the pattern wasconsistent with that of monoclinic Gd₂ O₃ and no pattern of cubic Gd₂ O₃was observed. It was confirmed that the starting phosphor was almostcompletely converted to the monoclinic Gd₂ O₃ :Eu phosphor. The powderbrightness of the obtained phosphor was 95% based on that of thestarting phosphor as measured when the phosphor was excited byultraviolet radiation of 254 nm in wavelength.

Subsequently, the obtained phosphor was coated onto the inner surface ofa glass tube using nitrocellulose binder, thereby forming a fluorescentlamp having a rated power of 40 W. Further, the same lamp as above wasformed using the starting phosphor. The luminous fluxes of bothfluorescent lamps were measured under the rated input. As a result, thefluorescent lamp of Example 3 exhibited a 3% higher value than that madeof the starting phosphor.

(Example 4)

A Gd₂ O₃ :Eu phosphor belonging to a cubic crystalline system was usedas a raw material. The average particle size of the starting phosphorwas 3.4 μm as measured by the Blaine method. The starting phosphor wassupplied into a high frequency plasma torch with a mixed gas of argonand oxygen as a carrier gas, fused, and quenched, thereby obtaining thephosphor powder sample. The phosphor powder was suspended in water,subjected to ultrasonic vibration, and allowed to stand still. After thesupernatant was removed, followed by vacuum filtration, the resultantmaterial was dried at 100° C., thereby obtaining the phosphor of thepresent invention. The average particle size of the obtained phosphorwas 3.6 μm as measured by the Blaine method. An electron microphotographof the obtained phosphor is shown in FIG. 7. The ratio of the majordiameter to the minor diameter with respect to individual phosphorparticles fell within the range of 1.00 to 1.10 as measured from theelectron microphotograph. The phosphor contained ultrafine particleshaving a size of 0.2 μm or less in an amount of 0.02 wt %. The X-raydiffraction pattern of this phosphor exhibited that of a monoclinicsystem which was entirely different from that of the starting phosphor.The emission spectrum of the phosphor was measured while the phosphorwas excited by an electron beam under an acceleration voltage of 10 kVand a current density of 1 μA/cm² or by ultraviolet radiation having awavelength of 254 nm. The results were as follows: a main emissionwavelength: 623 nm; and chromaticity: x=0.63, y=0.35. These valuesvaried from those of starting phosphor: a main emission wavelength: 611nm; and chromaticity: x=0.62 and y=0.36.

Subsequently, using the obtained phosphor, a phosphor screen was formedwith a coating weight of 7 mg/cm² on the inner surface of a glass bulbby means of the settling method. An aluminum backing was provided and anelectron gun was installed, followed by evacuation and sealing, therebyobtaining a 7-inch projection cathode-ray tube. The brightness of theprojection cathode-ray tube was 3500 ft-L as measured under a voltage of29 kV and a beam current of 1500 μA. This value was 30% higher than thebrightness, 2700 ft-L, of a cathode-ray tube formed in the same manneras above using a monoclinic Gd₂ O₃ :Eu phosphor obtained by firing at1300° C. followed by quenching.

(Example 5)

A Y₂ O₃ :Eu phosphor belonging to a cubic crystalline system was used asa raw material. The molar ratio of Eu/Y was 4.4%. The average particlesize of the starting phosphor was 3.2 μm as measured by the Blainemethod. The starting phosphor was supplied into a high frequency plasmatorch with a mixed gas of argon and oxygen as a carrier gas, fused,quenched, and recovered by a cyclone, thereby obtaining the phosphorconsisting of near spherical particles. The phosphor was suspended inwater, subjected to ultrasonic vibration, and allowed to stand still.After the supernatant was removed, followed by vacuum filtration, theresultant phosphor was dried. The resultant phosphor contained a traceamount of phosphor particles belonging to the monoclinic system besidesthe cubic system. In addition, this phosphor contained ultrafineparticles having a size of 0.2 μm or less in an amount of 0.1%. Thephosphor was further fired for 2 hours at 1100° C. in air. The resultantphosphor consisted of particles of cubic system alone. The averageparticle size of the obtained phosphor was 3.8 μm as measured by theBlaine method. The ratio of the major diameter to the minor diameterwith respect to individual phosphor particles fell within the range of1.00 to 1.10 as measured from an electron microphotograph. The ultrafineparticles were fused to some degree, and grew to deposit onto thesurface of the phosphor particles. The deposited amount wasapproximately 0.1%. The molar ratio of Eu/Y was 3.5%. The emissionspectrum of the phosphor was measured while the phosphor was excited byan electron beam under an acceleration voltage of 10 kV and a currentdensity of 1 μA/cm² or by ultraviolet radiation having a wavelength of254 nm. The main emission wavelength was 611 nm, which was consistentwith that of the starting phosphor. However, the luminous efficiency ofthe obtained phosphor was 110% as excited by the electron beam and 80%as excited by the ultraviolet radiation, based on those of the startingphosphor.

Subsequently, using the obtained phosphor, a phosphor screen was formedwith a coating weight of 7 mg/cm² on the inner surface of a glass bulbby means of the settling method. An aluminum backing was provided and anelectron gun was installed, followed by evacuation and sealing, therebyobtaining a 7-inch projection cathode-ray tube. There were nopeeling-off from the screen formed by the settling, and no problem suchas poor resistance to breakdown caused by vibration after completion ofcathode-ray tube formation. The brightness of the projection cathode-raytube was 5300 ft-L as measured under a voltage of 29 kV and a beamcurrent of 1500 μA. This value was 13% higher than the brightness, 4700ft-L, of a cathode-ray tube formed in the same manner as above using thestarting phosphor before subjecting to the thermal plasma treatment.

(Example 6)

Onto a color cathode-ray tube panel of 25 inch, phosphor stripes wereprovided using commercially available non-spherical blue-emittingphosphor and green-emitting phosphor. The phosphor formed in Example 4as a red-emitting phosphor was coated on the panel by a conventionalmethod. The coating weight was 4.0 mg/cm² as measured after the phosphorscreen was subjected to the ultraviolet-light exposure and developmentprocesses. When the sharpness of the stripe edges on the phosphor screenwas visually observed, it earned a maximum score of 10 points. When themonoclinic Gd₂ O₃ :Eu phosphor obtained by firing at 1300° C. followedby quenching was used, the coating weight was 3.1 mg/cm², and thesharpness score was 7 points. Subsequently, organic material filming,aluminum film deposition and baking were performed. A funnel and anelectron gun were attached, followed by evacuation and sealing, therebymanufacturing a cathode-ray tube. The brightness of red-emission of thecathode-ray tube was 120% based on that of a cathode-ray tube formed inthe same manner as above, using the monoclinic Gd₂ O₃ :Eu phosphor whichwas obtained by firing at 1300° C. followed by quenching.

It was determined that a range of color reproduction of the cathode-raytube is wider by about 7% as compared to that of either the cubic Gd₂ O₃:Eu that is the starting phosphor in Example 4 or the cubic Y₂ O₃ :Euthat is the starting phosphor in Example 5.

(Example 7)

The Gd₂ O₃ :Eu phosphor belonging to a cubic crystalline system as thesame as used in Example 4 was used as a raw material. The startingphosphor was supplied into a direct-current plasma torch for use in aplasma spraying with an argon gas as a carrier gas, fused, and quenchedby injecting into water, thereby obtaining the phosphor of the presentinvention. The average particle size of the obtained phosphor was 4.2 μmas measured by the Blaine method. The phosphor contained ultrafineparticles of 0.2 μm or less in an amount of 0.05 wt %. An electronmicrophotograph of the obtained phosphor is shown in FIG. 8. Thephosphor was slightly inferior in the spherical degree to that preparedby the high frequency thermal plasma method in Example 4. The ratio ofthe major diameter to the minor diameter with respect to individualparticles fell within the range of 1.00 to 1.30. The phosphor particleshaving the value in the above range were substantially regarded as nearspherical. The X-ray diffraction pattern of this phosphor, which wasentirely different from that of the starting phosphor, exhibited that ofa monoclinic system.

Subsequently, a phosphor screen was formed with a coating weight of 7mg/cm² on the inner surface of a glass bulb by means of the settlingmethod using the obtained phosphor. An aluminum backing was provided andan electron gun was installed, followed by evacuation and sealing,thereby obtaining a 7-inch projection cathode-ray tube. The brightnessof the projection cathode-ray tube was 3400 ft-L as measured under avoltage of 29 kV and a beam current of 1500 μA. This value was 6% higherthan the brightness, 3200 ft-L, of a cathode-ray tube formed in the samemanner as above using the starting phosphor.

(Example 8)

A Gd₂ O₃ :Tb phosphor containing 5 mol % of Tb and belonging to a cubiccrystalline system was used as a raw material for the thermal plasmatreatment. The average particle size of the starting phosphor was 3.5μm. The starting phosphor was supplied into a high frequency plasmatorch with an argon gas as a carrier gas, fused, quenched and recoveredby means of a cyclone, thereby obtaining the phosphor of the presentinvention. The average particle size of the obtained phosphor was 4.2μm. The ratio of the major diameter to the minor diameter with respectto individual phosphor particles fell within the range of 1.00 to 1.10as measured from an electron microphotograph. The phosphor containedultrafine particles of a size of 0.2 μm or less in an amount of 0.2 wt%. The X-ray diffraction pattern of this phosphor was of a monoclinicsystem which was entirely different from that of the starting phosphor.

Subsequently, when the phosphor was excited by an electron beam under anacceleration voltage of 10 kV and a current density of 1 μA/cm², theemission was green. The luminous efficiency was three times higher thanthat of the cubic starting phosphor.

Thereafter, a 7-inch projection cathode-ray tube was formed in the samemanner as in Example 4. The brightness of the projection cathode-raytube was 3.5 times higher than that of a cathode-ray tube formed usingthe starting phosphor under a voltage of 29 kV and a beam current of1500 μA.

(Example 9)

A Gd₂ O₃ :Pr phosphor belonging to a cubic crystalline system was usedas a raw material for the thermal plasma treatment. The average particlesize of the starting phosphor was 3.2 μm as measured by the Blainemethod. The starting phosphor was supplied into a high frequency plasmatorch with an argon gas as a carrier gas, fused, and quenched, therebyobtaining the phosphor of the present invention. The average particlesize of the obtained phosphor was 3.8 μm as measured by the Blainemethod. The ratio of the major diameter to the minor diameter withrespect to individual phosphor particles fell within the range of 1.00to 1.10 as measured from an electron microphotograph. The X-raydiffraction pattern of this phosphor exhibited a monoclinic system whichwas entirely different from that of the starting phosphor. The emissionspectrum of the phosphor was measured while the phosphor was excited byan electron beam under an acceleration voltage of 10 kV and a currentdensity of 1 μA/cm² or by ultraviolet radiation having a wavelength of254 nm. Consequently, the emission color was green, and chromaticitythereof was x=0.31 and y=0.51. These values varied from those ofstarting phosphor exhibiting red emission color and chromaticity ofx=0.64 and y=0.28.

Subsequently, a phosphor screen was formed with a coating weight of 7mg/cm² on the inner surface of a glass bulb by means of the settlingmethod using the obtained phosphor. An aluminum backing was provided andan electron gun was installed, followed by evacuation and sealing,thereby obtaining a 7-inch projection cathode-ray tube. The brightnessof the projection cathode-ray tube was 580 ft-L as measured under avoltage of 29 kV and a beam current of 1500 μA. This value was 16%higher than the brightness, 500 ft-L, of a cathode-ray tube formed inthe same manner as above using a monoclinic Gd₂ O₃ :Pr phosphor whichwas obtained by firing at 1300° C. followed by quenching.

(Comparative Example 1)

A slurry was prepared by blending a commercially available divalenteuropium-activated barium calcium strontium halophosphate phosphor, acommercially available cerium terbium-activated lanthanum phosphatephosphor, and a commercially available europium-activated yttrium oxidephosphor whose shape was not spherical. The slurry was coated on theinner surface of a glass tube having 32 mm in diameter to form a 40 Wconventional three-band type straight fluorescent lamp havingchromaticity on black body locus at a color temperature of 5000 K. Totalluminous flux at the lighting-up time of zero was 3640 lumen, and aspecific color rendering index of R₉ was 35.

(Comparative Example 2)

A co-precipitate of gadolinium and europium oxalate was fired todecompose at 900° C., the resultant product was fired at 1400° C. usingan alkali earth halide as a flux, thereby obtaining a Gd₂ O₃ :Euphosphor containing 5 mol % of Eu. By measuring the X-ray diffraction,it was found that the starting phosphor is converted to an almostcomplete monoclinic system. The average particle size of the phosphorwas 3.5 μm as measured by the Blaine method.

Subsequently, three types of phosphors described in Comparative Example1 were blended with the Gd₂ O₃ :Eu phosphor and then a 5000 K and 40 Wstraight fluorescent lamp was formed. Total luminous flux at thelighting-up time of zero was 3580 lumen, and the specific colorrendering index of R₉ was 47.

As compared to the fluorescent lamp of Comparative Example 1, the totalluminous flux was lower by 1.6%, but R₉ was higher by 12 points. Thisdata demonstrated that improvement in R₉ by 10 points inevitablyaccompanied a decrease in the total luminous flux by 1.4%.

(Example 10)

A co-precipitate of gadolinium and europium oxalate as the same as usedin Comparative Example 2 was fired to decompose at 1000° C., therebyobtaining a Gd₂ O₃ :Eu powder. The obtained powder exhibited adiffraction pattern of a cubic system as measured by the x-raydiffraction. Thereafter, the powder was supplied into a high frequencyplasma torch with a mixed gas of argon and oxygen as a carrier gas,fused and quenched, thereby obtaining the phosphor of the presentinvention. The average particle size of the phosphor was 1.5 μm asmeasured by the Blaine method. An electron microphotograph of theobtained phosphor is shown in FIG. 9. The ratio of the major diameter tothe minor diameter with respect to individual phosphor particles fellwithin the range of 1.00 to 1.15 as measured from the electronmicrophotograph. From the ratio of the X-ray diffraction peaks of thisphosphor, the ratio of the cubic system to the monoclinic system thereofwas calculated. It was found that the monoclinic system was contained inan amount of approximately 80%.

Subsequently, three types of phosphors in Comparative Example 1 wereblended with the Gd₂ O₃ :Eu phosphor of the present invention and a 5000K and 40 W straight fluorescent lamp was formed. The Gd₂ O₃ :Eu phosphorwas blended in a ratio of 20 wt %. Total luminous flux at thelighting-up time of zero was 3570 lumen, and a specific color renderingindex for red color R₉ was 55. As compared to the fluorescent lamp ofComparative Example 1, the total luminous flux was lower by 2.0%, but R₉was higher by 20 points. This data demonstrated that to improve R₉ by 10points the total luminous flux decreases by only 1.0%.

(Example 11)

As a raw material, use was made of Y₂ O₂ S:Eu formed by a flux method asthe same as red-emitting phosphor for use in a color TV. The molar ratioof Eu/Y was set to 8.0%. The average particle size of the startingphosphor was 4.1 μm. The starting phosphor was washed while beingstirred in a 1/40 diluted aqueous nitric acid solution for 20 minutes,followed by vacuum filtration. After water was replaced with alcohol,the resultant phosphor was dried. To the obtained sample, sulfur wasadded in an amount of 2 wt %, and then the sample was introduced into a4 MHz high frequency plasma torch under an argon atmosphere, quenchedand recovered by a cyclone. To the resultant sample placed in water, anultrasonic vibration was applied. After the sample was allowed to standstill, the supernatant was removed, thereby obtaining sphericalparticles. On the surface of the spherical particles, ultrafineparticles of approximately 0.1 μm in size was contained in an amount of0.05 wt %. The body color of the spherical particles was grey-purple.The reflectance for visible light was 40%. Further, the sphericalparticles were fired for one hour at 900° C. in an sulfur atmosphere,thereby obtaining the phosphor of the present invention. An electronmicrophotograph is shown in FIG. 10. The phosphor consisted of sphericalparticles having an average particle size of 4.5 μm. The ratio of themajor diameter to the minor diameter with respect to individual phosphorparticles fell within the range of 1.00 to 1.10 as measured from theelectron microphotograph. The body color of the phosphor was white andthe reflectance for visible light thereof was 94%. The X-ray diffractionpattern of this phosphor was consistent with that of an oxysulfidecompound. The molar ratio of Eu/Y of the phosphor was 3.7%. When thephosphor was excited by an electron beam under an accelerating voltageof 10 kV and a current density of 0.5 μA/cm², the emission color was redsuitable for a color TV.

Onto a color cathode-ray tube panel of 25 inch, phosphor stripes wereformed by using commercially available non-spherical blue-emitting andgreen-emitting phosphor, and then the phosphor obtained in Example 11serving as a red-emitting phosphor was coated by the conventionalmethod. The transmittance of the phosphor layer for the ultraviolet rayhaving wavelength of 420 to 350 nm for exposure was 3%. After thephosphor screen was subjected to exposure and development, the coatingweight was 4.0 mg/cm². When the sharpness at edge portions of phosphorstripes was visually observed, it earned a maximum score of 10 points.In contrast, the phosphor layer formed of a red-emitting phosphor foruse in a color TV with no thermal plasma treatment exhibitedtransmittance for the ultraviolet ray of 1%, a coating weight of 3.5mg/cm², and a sharpness score of 9 points. Subsequently, organicmaterial filming, aluminum film deposition and baking were performed. Afunnel and an electron gun were attached, followed by evacuation andsealing, thereby manufacturing a cathode-ray tube. The red-emissionbrightness of the cathode-ray tube was 120% based on that of acathode-ray tube formed in the same manner as above using a Y₂ O₂ S:Eured-emitting phosphor having an Eu/Y molar ratio of 3.7%, which was notplasma-treated.

(Comparative Example 3)

As a raw material, use was made of a red-emitting phosphor Y₂ O₂ S:Eufor use in a color TV. The molar ratio of Eu/Y was 4.1%. The averageparticle size of the starting phosphor was 4.3 μm. The starting phosphorwas washed while being stirred in a 1/40 diluted aqueous nitric acidsolution for 20 minutes, followed by vacuum filtration. After water wasreplaced with alcohol, the resultant phosphor was dried. The obtainedsample was introduced into a 4 MHz high frequency plasma torch andquenched. To the resultant sample, an ultrasonic vibration was appliedin water and allowed to stand still. After removing the supernatant,spherical particles were obtained. The body color of the sphericalparticles was grey-purple. The reflectance for visible light was 8%.Further, this sample was fired for one hour at 900° C. in a sulfuratmosphere in the same manner as in Example 11. The body color of thisphosphor was white and the molar ratio of Eu/Y was 1.8. The emissioncolor under an electron excitation was orange, which was not suitablefor use in a color TV.

(Example 12)

As a raw material, use was made of Y₂ O₂ S:Tb having an average particlesize of 1.5 μm formed by the flux method. The molar ratio of Tb/Y was6.5%. To the starting phosphor, a 1/100 diluted aqueous Tamol solutionwas added, followed by vacuum filtration. After water was replaced withalcohol, the resultant phosphor was dried. To the obtained sample,sulfur was added in an amount of 3 wt %, and then the sample wasintroduced into a 4 MHz high frequency plasma torch under an argonatmosphere, quenched and recovered by a cyclone. To the resultantsample, an ultrasonic vibration was applied in water and allowed tostand still. After removing the supernatant, spherical particles wereobtained. The obtained phosphor contained ultrafine particles in anamount of 0.05%. The body color of the sample exhibited human-skincolor. The reflectance for visible light was 50%. Further, the samplewas fired for one hour at 900° C. in a sulfur atmosphere in the samemanner as in Example 11, thereby obtaining the phosphor of the presentinvention. The phosphor comprised white spherical particles having anaverage particle size of 1.2 μm and detectable ultrafine particles in anamount of 0.02%. The reflectance for visible light of the phosphor was91% and the molar ratio of Tb/Y was 3.5%. In the emission spectrum byelectron excitation, the intensity of a 544 nm band was 10 timesstronger than that of a 415 nm band, and green emission was resulted.

This phosphor was deposited on a glass substrate having a diameter of 25mm by the settling method using potassium sulfate and potassiumsilicate, thereby forming a phosphor layer. Subsequently, an organicmaterial filming, aluminum film deposition and baking were performed.The obtained phosphor screen on the glass substrate was mounted to aglass bulb for X-ray image intensifier having a 9-inch input screen asan output screen, and the bulb was evacuated and sealed, therebyobtaining an x-ray image intensifier. When the x-ray image intensifierwas driven in a cathode voltage of 25 kV, the resolution on the outputscreen was 55 line pairs/cm in a center thereof and the light output was80 nit per 1 mR/sec of X-ray input. In a similar way, an X-ray imageintensifier was manufactured using the starting Y₂ O₂ S:Tb phosphor,consisting of the non-spherical particles. The resolution of this X-rayimage intensifier was 40 line pairs/cm and the light output was 75 nit.

(Example 13)

Each of the Y₂ O₃ :Eu phosphor used as a raw material in Example 1 andthe spherical Y₂ O₃ :Eu phosphor in Example 1 was individually coated onan inner surface of a glass tube having an inner diameter of 4.5 mm in acoating weight 6 mg/cm² by the sucking method under reduced pressure,thereby forming cold cathode fluorescent lamps having a length of 150mm. 50% of the outer surface of the lamp was covered with a reflectingfilm having reflectance of 80%. When these fluorescent lamps were turnedon under an input power of 2 W, the difference in brightness betweenboth ends of the fluorescent lamp using the starting phosphor of Example1 was 6%; whereas, that of the fluorescent lamp using the sphericalphosphor was 1.5%.

Subsequently, as shown in a liquid crystal display of FIG. 4,red-emitting monochrome liquid crystal display devices were manufacturedby using the fluorescent lamps mentioned above in combination with areflecting film 32 and a monochrome TFT liquid crystal panel without acolor filter. Under an input powder of 2 W, the brightness of thedisplay surface in the case of the fluorescent lamp using the sphericalphosphor was 18% higher than that of the fluorescent lamp using thestarting phosphor of Example 1.

(Example 14)

A fluorescent lamp having the same shape as in Example 13 is formed in asame manner as in Example 13 except that a phosphor mixture containingcommercially available blue-, green- and red-emitting phosphors for afluorescent lamp are blended and that the coating weight is set to 5.5mg/cm². When the obtained fluorescent lamp is turned on under an inputpower of 2 W, the difference in brightness between both ends of thefluorescent lamp is 9%. A diffuse transmittance of a phosphor layer ofthe fluorescent lamp is 30%.

Subsequently, each of the commercially available blue-, green- andred-emitting phosphors is supplied into a high frequency plasma with anargon gas as a carrier gas, fused, quenched, and washed with anultrasonic vibration in water, thereby obtaining three types ofspherical phosphors having average particle sizes of 4.6 μm (the contentof ultrafine particles having a diameter of 0.2 μm or less: 0.08 wt %),5.2 μm (the content of ultrafine particles having a diameter of 0.2 μmor less: 0.06 wt %) and 4.3 μm (the content of ultrafine particleshaving a diameter of 0.2 μm or less: 0.01 wt %), respectively. Thesimilar fluorescent lamp as above is manufactured by using a mixture ofthese phosphors. Under an input power of 2 W, the difference inbrightness between both ends of the fluorescent lamp is 2.5%. Thediffuse transmittance of a phosphor layer of the fluorescent lamp is55%, which is 1.8 times higher than that of the former fluorescent lamp.

A color liquid crystal display is manufactured in the same manner as inExample 13 except that each of two types of fluorescent lamps and a TFTliquid crystal panel having a color filter are used. Each of the liquidcrystal displays is driven to show a white color by turning on eachfluorescent lamp under an input power of 2 W. The brightness of thedisplay surface of the latter display having the fluorescent lamp usingspherical phosphors is 12% higher than that of the former display havingthe fluorescent lamp using the starting phosphor.

(Example 15)

A starting CaWO₄ phosphor having an average particle size of 11.3 μm wasprepared by the conventional wet precipitation and firing method. Thestarting phosphor was supplied into a high frequency plasma with a mixedgas of argon and oxygen as a carrier gas, fused and quenched, therebyobtaining the phosphor of the present invention. The average particlesize of the obtained phosphor was 10.5 μm. After an ultrasonic vibrationwas applied, the resultant phosphor contained ultrafine particles havinga diameter of 0.2 μm or less in an amount of 0.05 wt %. To thisphosphor, a binder was added, thereby preparing a slurry. The obtainedslurry was uniformly coated on a screen base by means of the doctorblade method so as to obtain a phosphor coating weight of 40 mg/cm²after drying, thereby forming a phosphor layer. Thereafter a protectionfilm was adhered thereon to obtain a radiation intensifying screen(Example 15). For comparison, a radiation intensifying screen(Comparative Example 4) was obtained in the same manner as above byusing the starting phosphor. The thickness of a phosphor layer of eachradiation intensifying screen was measured. As a result, it was 149 μmin Comparative Example 4, whereas, 128 μm in Example 15.

Subsequently, each obtained intensifying screen was allowed to superposeon X-ray film, and an X-ray photograph was taken according to theconventional method. Sensitivity and sharpness of developed X-ray filmwere evaluated. The sensitivity of Example 15 was 104% based on a valueof Comparative Example 4. MTF was measured by the contrast method. Thesharpness was compared by using the value of MTF spatial frequency at 2line pairs/mm. The sharpness of Example 15 was 109% based on the valueof Comparative Example 4.

(Example 16)

A starting Gd₂ O₂ S:Tb phosphor having an average particle size of 4.9μm was prepared by the conventional firing method with fluxes. Thestarting phosphor was supplied into a high frequency plasma with argonas a carrier gas, fused and quenched, thereby obtaining the phosphorhaving an average particle size of 2.4 μm of the present invention.Further, the phosphor of the present invention having an averageparticle size of 9.5 μm was obtained in the same manner as mentionedabove by changing conditions of the plasma and of the powder supplier.Phosphors obtained after applying the ultrasonic vibration and thedrying, were further fired in a sulfur atmosphere. The obtainedphosphors contained ultrafine particles having a diameter of 0.2 μm orless in an amount of 0.05 wt % and 0.01 wt %, respectively. To eachphosphor, a binder was added, thereby preparing two types of phosphorslurries. The obtained two slurries were successively coated on a screenbase uniformly by the doctor blade method so as to provide adouble-layered phosphor layer having a thickness of 250 μm after drying.Thereafter a protection film was adhered thereon to obtain a radiationintensifying screen (Example 16). For comparison, a radiationintensifying screen (Comparative Example 5) was manufactured in the samemanner as above using two phosphors, prepared by the conventionalmethod, having the average particle sizes of 2.5 μm and 9.8 μm,respectively. A coating weight of each intensifying screen was measured.It was 78 mg/cm² in Comparative Example 5 and 96 mg/cm² in Example 16.

Subsequently, each obtained intensifying screen was allowed to superposeon X-ray film, and an X-ray photograph was taken according to aconventional method. Sensitivity and sharpness of developed X-ray filmwere evaluated. The sensitivity of Example 16 was 109% based on a valueof Comparative Example 5. MTF was measured by a contrast method. Thesharpness was measured by using the value of MTF spatial frequency at 2line pairs/mm. The sharpness of Example 16 was 101% based on the valueof Comparative Example 5.

(Example 17)

A starting Gd₂ O₂ S:Tb phosphor having an average diameter of 8.1 μm wasprepared by the conventional firing method with fluxes. The startingphosphor was supplied into a high frequency plasma with argon as acarrier gas, fused and quenched. The resultant phosphor was furtherfired at 700° C. in a sulfur atmosphere, thereby obtaining the phosphorhaving an average particle size of 7.6 μm. After applying the ultrasonicvibration, the resultant phosphor contained ultrafine particle having adiameter of 0.2 μm or less in an amount of 1 wt %. The phosphor wascoated on a protection film by the settling method so as to obtain acoating weight of 80 mg/cm² after drying. Thereafter, a screen base wasadhered thereon to obtain a radiation intensifying screen. A test pieceof 1 cm² in size was cut from the radiation intensifying screen, and thecross section was observed with SEM. It was found that the obtainedintensifying screen consisted of a layer containing ultrafine particlesalone on the side of the screen base and a layer containing bothultrafine particles and spherical particles on the former layer.

Subsequently, the obtained intensifying screen was allowed to superposeon X-ray film, and an X-ray photograph was taken according to aconventional method. Sensitivity and sharpness of the developed X-rayfilm were evaluated. The sensitivity of Example 17 was 112% based on avalue of Comparative Example 5. MTF was measured by the contrast method.The sharpness was compared by using the value of MTF spatial frequencyat 2 line pairs/mm. The sharpness of Example 17 was 118% based on thevalue of Comparative Example 5.

(Example 18)

As a raw material, use was made of Gd₂ O₂ S:Pr having an averageparticle size of 5.2 μm prepared by the flux method. The molar ratio ofPr/Gd was 0.06%. To the starting phosphor, a 1/100 diluted aqueous Tamolsolution was added, followed by vacuum filtration. After water wasreplaced with alcohol, the resultant phosphor was dried. The obtainedsample was introduced into a 4 MHz high frequency plasma torch under anargon atmosphere and quenched. The resultant phosphor containedultrafine particles in an amount of 1%. To the sample, an ultrasonicvibration was applied in water and allowed to stand still. Afterremoving the supernatant, spherical particles containing ultrafineparticles in an amount of 0.1% were obtained. The body color of thephosphor exhibited human-skin color. The reflectance for visible lightwas 32%. Further, the phosphor was fired for one hour at 900° C. in asulfur atmosphere in the same manner as in Example 11, thereby obtainingthe phosphor of the present invention. On the surface of the phosphor,approximately 0.1% of the ultrafine particles were remained in a fusedform. The phosphor consists of white spherical particles having anaverage particle size of 6.1 μm. The reflectance for visible light ofthe phosphor was 93%. When the X-ray diffraction of the phosphor wasmeasured, it exhibited a diffraction pattern of an oxysulfide. The molarratio of Pr/Gd was 0.05%. The emission color by electron excitation wasgreen which was consistent with that of the starting phosphor.

(Example 19)

As a raw material, use was made of Y₂ O₂ S:Tb having an average particlesize of 4.3 μm prepared by the flux method. The molar ratio of Tb/Y was6.5%. To the starting phosphor, a 1/100 diluted aqueous Tamol solutionwas added, followed by vacuum filtration. After water was replaced withalcohol, the resultant phosphor was dried. To the obtained sample,sulfur was added in an amount of 3 wt %, and then the sample wasintroduced into a 4 MHz high frequency plasma torch under an argonatmosphere, quenched and recovered by a cyclone. To the resultantsample, an ultrasonic vibration was applied in water and allowed tostand still. After removing the supernatant, spherical particles wereobtained. The obtained phosphor contained ultrafine particles in anamount of 0.05%. The body color of the phosphor exhibited human-skincolor. The reflectance for visible light was 50%. Further, the sphericalparticles were fired for one hour at 900° C. in a sulfur atmosphere inthe same manner as in Example 11, thereby obtaining the phosphor of thepresent invention. The phosphor consists of white spherical particleshaving an average particle size of 5.5 μm containing detectableultrafine particles in an amount of 0.02%. The reflectance for visiblelight of the phosphor was 91% and the molar ratio of Tb/Y was 3.5%. Inthe emission spectrum by electron excitation, the intensity of the 544nm band was 10 times stronger than that of the 415 nm band, and theemission color was green suitable for a phosphor for use in a projectioncathode-ray tube.

(Comparative Example 6)

CaWO₄ phosphor (Comparative Example 6) was prepared by a conventionalwet precipitation and firing method. The average particle size of theobtained phosphor was 4.3 μm as measured by the Blaine method. The peakwavelength of the phosphor in the emission spectrum was 411 nm asexcited by ultraviolet radiation or electron beams. Chromaticity valuesare as follows: x=0.165 and y=0.120.

(Example 20)

A starting CaWO₄ phosphor of Comparative Example 6 was supplied into ahigh frequency plasma with a mixed gas of argon and oxygen as a carriergas, fused and quenched, thereby obtaining a phosphor (Example 20). Theaverage particle size of the phosphor was 3.9 μm as measured by theBlaine method. A microphotograph of the obtained phosphor is shown inFIG. 11. The ratio of the major diameter to the minor diameter withrespect to individual phosphor particles fell within the range of 1.00to 1.08 as measured from the electron microphotograph. Phosphorparticles of 0.2 μm or less generated by partial evaporation of thephosphor were deposited on the surface of the spherical particles. Afterapplying the ultrasonic vibration and removing the supernatant, theresultant phosphor contained ultrafine particles of 0.2 μm or less in anamount of 0.1 wt %. It was found that the X-ray diffraction pattern ofthis phosphor was consistent with that of CaWO₄.

The emission spectrum of the phosphor was determined by ultraviolet orelectron beam excitation. As a result, the peak wavelength was 433 nm,which was shifted by 20 nm or more toward the longer wavelength sidecompared to the spectrum of the phosphor of Comparative Example 6.Hence, the chromaticity thereof was x=0.173 and y=1.44. The powderbrightness was 78% based on the value of the phosphor of ComparativeExample 6, as measured by exciting with ultraviolet ray of 254 nm. Also,the powder brightness of the phosphor was measured by electron beamexcitation under a current density of 0.5 μA/cm² and an acceleratingvoltage of 10 kV. As a result, it exhibited approximately 102% based onthat of the phosphor of Comparative Example 6.

Subsequently, the obtained phosphor was coated on a glass substrate bythe settling method, thereby forming a phosphor layer having a coatingweight of 10 mg/cm². The transmittance of the phosphor layer was 1.7times higher than that formed of the phosphor Comparative Example 6.

Further, a phosphor screen having a coating weight of 6 mg/cm² wasformed on the inner surface of a glass bulb using the obtained phosphorby the settling method. An aluminum backing was provided and an electrongun was installed, followed by evacuation and sealing, thereby obtaininga 7 inch cathode-ray tube. The brightness of the cathode-ray tube wasmeasured under an accelerating voltage of 30 kV and a beam current of500 μA. This value was 118% based on a value of a cathode-ray tubeformed in the same manner as above using the phosphor of ComparativeExample 6.

(Comparative Example 7)

CaWO₄ :Pb phosphor (Comparative Example 7) was prepared by theconventional wet precipitation and firing method. The average particlesize of the phosphor was 3.6 μm as measured by the Blaine method. Thepeak wavelength in the emission spectrum of the phosphor was 435 nm asdetermined by ultraviolet radiation or electron beam excitation.Chromaticity values were as follows: x=0.172 and y=1.169. The peakwavelength in the excitation spectrum positioned at 270 nm.

(Example 21)

A starting CaWO₄ :Pb phosphor of Comparative Example 7 was supplied intoa high frequency plasma with a mixed gas of argon and oxygen as acarrier gas, fused and quenched, thereby obtaining a phosphor (Example21). The average particle size of the phosphor was 3.1 μm as measured bythe Blaine method. The ratio of the major diameter to the minor diameterwith respect to individual phosphor particles fell within the range of1.00 to 1.11 as measured from an electron microphotograph. Afterapplying the ultrasonic vibration and removal of the supernatant, theresultant phosphor contained ultrafine particles of 0.2 μm or less in anamount of 0.05 wt %. It was found that the X-ray diffraction pattern ofthis phosphor was consistent with that of CaWO₄.

The emission spectrum of the phosphor was determined by ultravioletexcitation or electron beam excitation. As a result, the peak wavelengthwas 458 nm. Chromaticity values thereof were as follows: x=0.180 andy=1.86. The main peak of the excitation spectrum laid at 259 nm, ofwhich deviation from 254 nm was little. The powder brightness was 105%based on the value of the phosphor of Comparative Example 7, as measuredby exciting with an ultraviolet ray of 254 nm. Also, the powderbrightness of the phosphor was measured by electron beam excitationunder a current density of 0.5 μA/cm² and an accelerating voltage of 10kV. As a result, it exhibited approximately 103% based on that of thephosphor of Comparative Example 7.

Subsequently, the phosphor was coated on a glass substrate by thesettling method, thereby forming a phosphor layer having a coatingweight of 9 mg/cm². The transmittance of the phosphor layer was 1.8times higher than that formed of the phosphor of Comparative Example 7.

Further, a phosphor screen having a coating weight of 6 mg/cm² wasformed on the inner surface of a glass bulb using the obtained phosphorby the settling method. An aluminum backing was provided and an electrongun was installed, followed by evacuation and sealing, thereby obtaininga 7 inch cathode-ray tube. The brightness of the cathode-ray tube wasmeasured under an accelerating voltage of 30 kV and a beam current of500 μA. The value was 121% based on a value of a cathode-ray tube formedin the same manner as above using the phosphor of Comparative Example 7.

(Comparative Example 8)

A MgWO₄ :Pb phosphor (Comparative Example 8) was prepared by theconventional firing method. The average particle size of the phosphorwas 4.2 μm as measured by the Blaine method. A peak wavelength of thephosphor in the emission spectrum was 498 nm as measured by ultravioletradiation or electron beam excitation. Chromaticity values were asfollows: x=0.225 and y=0.418.

(Example 22)

A starting MgWO₄ phosphor of Comparative Example 8 was supplied into ahigh frequency plasma with a mixed gas of argon and oxygen as a carriergas, fused and quenched, thereby obtaining a phosphor (Example 22). Theaverage particle size of the phosphor was 4.0 μm as measured by theBlaine method. The ratio of the major diameter to the minor diameterwith respect to individual phosphor particles fell within the range of1.00 to 1.07 as measured from an electron microphotograph. After theultrasonic vibration was applied and the supernatant was removed, theresultant phosphor contained ultrafine particles of 0.2 μm or less in anamount of 0.2 wt %. It was found that the X-ray diffraction pattern ofthis phosphor was consistent with that of MgWO₄.

The emission spectrum of the phosphor was determined by ultraviolet orelectron beam excitation. As a result, the peak wavelength was 512 nm.Chromaticity values thereof were as follows: x=0.233 and y=0.441. Theexcitation spectrum was shifted toward the shorter wavelength side of254 nm. The powder brightness was 114% based on the value of thephosphor of Comparative Example 8, as measured by exciting withultraviolet ray of 254 nm. Also, the powder brightness of the phosphorwas measured by electron beam excitation under a current density of 0.5μA/cm² and an accelerating voltage of 10 kV. As a result, it exhibitedapproximately 109% based on that of the phosphor of Comparative Example8. As mentioned above, as a result of shifting the excitation spectrumtoward the shorter wavelength side, absorption of ultraviolet radiationincreased, resulting in high luminous efficiency.

Subsequently, the phosphor was coated on a glass substrate by thesettling method, thereby forming a phosphor layer having a coatingweight of 12 mg/cm². The transmittance of the phosphor layer was 1.5times higher than that formed of the phosphor Comparative Example 8.

Further, a phosphor screen having a coating weight of 6 mg/cm² wasformed on the inner surface of a glass bulb using the obtained phosphorby the settling method. An aluminum backing was provided and an electrongun was installed, followed by evacuation and sealing, thereby obtaininga 7 inch cathode-ray tube. The brightness of the cathode-ray tube wasmeasured under an accelerating voltage of 30 kV and a beam current of500 μA. The value was 115% based on a value of a cathode-ray tube formedin the same manner as above using the phosphor of Comparative Example 8.

(Example 23)

A CaWO₄ phosphor having an average particle size of 5.1 μm was preparedby the conventional firing method. The starting phosphor was suppliedinto a high frequency plasma with a mixed gas of argon and oxygen as acarrier gas, fused and quenched, thereby obtaining the phosphor havingan average particle size of 2.5 μm of the present invention. Further,the phosphor of the present invention having an average particle size of9.6 μm was obtained in the same manner as mentioned above by changingconditions in the plasma and the powder supplier. After applying theultrasonic vibration, these phosphors contained ultrafine particleshaving a diameter of 0.2 μm or less in an amount of 0.05 wt % and 0.01wt %, respectively. To each phosphor, a binder was added, therebypreparing two types of phosphor slurries. The obtained two slurries weresuccessively coated on a screen base uniformly by the doctor blademethod so as to provide a double-layered phosphor layer having athickness of 250 μm after drying. Thereafter a protection film wasadhered thereon to obtain a radiation intensifying screen. Forcomparison, a radiation intensifying screen (Comparative Example 9) wasmanufactured in the same manner as above using two phosphors, preparedby the conventional method, having average particle sizes of 2.7 μm and9.9 μm, respectively. The coating weight of each intensifying screen wasmeasured. It was 77 mg/cm² in Comparative Example 9 and 94 mg/cm² inExample 23.

Subsequently, each obtained intensifying screen was allowed to superposeon X-ray film, and an X-ray photograph was taken according to aconventional method. Sensitivity and sharpness of developed X-ray filmwere evaluated. The sensitivity of Example 23 was 108% based on a valueof Comparative Example 9. MTF was measured by a contrast method. Thesharpness was compared by using the value of MTF spatial frequency at 2line pairs/mm. The sharpness of Example 23 was 101% based on the valueof Comparative Example 4.

(Example 24)

A CaWO₄ phosphor having an average particle size of 8.3 μm was preparedby the conventional wet precipitation and firing method. The startingphosphor was supplied into a high frequency plasma with a mixed gas ofargon and oxygen as a carrier gas, fused and quenched, thereby obtainingthe phosphor of the present invention. An average particle size of theobtained phosphor was 7.8 μm. After the ultrasonic vibration was appliedand the supernatant was removed, the resultant phosphor containedultrafine particles of 0.2 μm or less in an amount of 1 wt %. Thephosphor was coated on a protection film by the settling method so as toobtain a coating weight of 50 mg/cm² after drying. Thereafter, a screenbase was adhered thereon to obtain a radiation intensifying screen. Atest piece of 1 cm² in size was cut from the radiation intensifyingscreen and the cross section was observed with SEM. As a result, it wasfound that the obtained intensifying screen consisted of a layercontaining ultrafine particles alone on the side of the screen base anda layer containing both ultrafine particles and spherical particles onthe former layer.

Subsequently, the obtained intensifying screen was allowed to superposeon X-ray film, an X-ray photograph was taken according to a conventionalmethod. Sensitivity and sharpness of developed X-ray film wereevaluated. The sensitivity of Example 24 was 135% based on a value ofComparative Example 4. MTF was measured by the contrast method. Thesharpness was compared by using the value of MTF spatial frequency at 2line pairs/mm. The sharpness of Example 24 was 111% based on the valueof Comparative Example 4.

What is claimed is:
 1. A phosphor, comprising:transparent sphericalparticles having an average particle size of 0.5 to 20 μm and a ratio ofthe major diameter to the minor diameter of individual particles in therange of 1.0 to 1.5; and ultrafine particles having a diameter of 0.2 μmor less in an amount of 0.001-5 wt % or less.
 2. The phosphor accordingto claim 1, represented by the formula:Ln₂ O₃ :Rwherein Ln is at leastone element selected from the group consisting of La, Gd, Lu and Y, andR is at least one element selected from the lanthanide group; andcomprising, transparent spherical particles having an average particlesize of 0.5 to 15 μm and a ratio of the major diameter to the minordiameter of individual particles in the range of 1.0 to 1.5, andultrafine particles having a diameter of 0.2 μm or less in an amount of0.001-2 wt % or less.
 3. The phosphor according to claim 2, wherein saidR is Eu and the molar ratio of Eu to Ln is 1 to 6%, said R is Tb and themolar ratio of Tb to Ln is 0.1 to 6%, or said R is Pr and the molarratio of Pr to Ln is 0.01 to 5%.
 4. The phosphor according to claim 2,represented by the formula:Gd₂ O₃ :Rwherein R is at least one elementselected from the lanthanide group; and at least part of crystallinesystem is a monoclinic system.
 5. The phosphor according to claim 4,wherein said R is Eu and the molar ratio of Eu to Gd is 1 to 6%; said Ris Tb and the molar ratio of Tb to Gd is 0.1 to 6%; or said R is Pr andthe molar ratio of Pr to Gd is 0.01 to 5%.
 6. The phosphor according toclaim 4, represented by the formula:Gd₂ O₃ :Eu,wherein the content ofthe monoclinic system is 5 to 100% and the average particle size is 0.5to 3 μm.
 7. The phosphor according to claim 1, represented by theformula:Ln₂ O₂ S:Rwherein Ln is at least one element selected from thegroup consisting of Y, La, Gd, and Lu, and R is at least one elementselected from the lanthanide group; and comprising, transparentspherical particles having an average particle size of 0.5 to 15 μm anda ratio of the major diameter to the minor diameter of individualparticles in the range of 1.0 to 1.5, and ultrafine particles having adiameter of 0.2 μm or less in an amount of 0.001-2 wt % or less.
 8. Thephosphor according to claim 7, wherein said R is Eu and the molar ratioof Eu to Ln is 2 to 7%; said R is Tb and the molar ratio of Tb to Ln is0.1 to 6%; or said R is Pr and the molar ratio of Pr to Ln is 0.01 to0.5%.
 9. The phosphor according to claim 7, having reflectance forvisual light of 85% or more when the phosphor is used in a layer havingan optically indefinite thickness.
 10. The phosphor according to claim1, represented by the formula:MWO₄ wherein M is at least one componentselected from the group consisting of Ca and Mg; or CaWO₄ :Pb,andcomprising, transparent spherical particles having an average particlesize of 0.5 to 20 μm and a ratio of the major diameter to the minordiameter of individual particles in the range of 1.0 to 1.5, andultrafine particles having a diameter of 0.2 μm or less in an amount of0.001-5 wt % or less.
 11. The phosphor according to claim 10, wherein acontent of said ultrafine particles having a diameter of 0.2 μm or lessis 0.01 to 2 wt %.